Low-pressure mercury vapor discharge lamp and compact fluorescent lamp

ABSTRACT

A low-pressure mercury vapor discharge lamp has a light-transmitting discharge vessel ( 10 ), enclosing, in a gastight manner, a discharge space ( 11 ) provided with a filling of mercury and a rare gas. The discharge vessel ( 10 ) comprises means ( 41   a ) for maintaining a discharge in the discharge space ( 11 ). At least a part of an inner wall of the discharge vessel ( 10 ) is provided with a protective translucent layer ( 16 ). According to the invention, the discharge vessel ( 10 ) is provided with a pinched seal ( 20 ). In addition, the translucent layer ( 16 ) comprises a borate and/or a phosphate of an alkaline earth metal and/or of scandium, yttrium or a further rare earth metal. Preferably, the glass composition is made from a sodium-rich glass including the following constituents: 70-75 wt. % SiO2, 15-18 wt. % Na2O, 0.25-2 wt. % K2O. The discharge lamp according to the invention has a comparatively high maintenance.

The invention relates to a low-pressure mercury vapor discharge lampcomprising a light-transmitting discharge vessel,

the discharge vessel enclosing, in a gastight manner, a discharge spaceprovided with a filling of mercury and a rare gas,

the discharge vessel comprising means for maintaining a discharge in thedischarge space,

while at least a part of an inner wall of the discharge vessel isprovided with a translucent layer.

The invention also relates to a compact fluorescent lamp.

In mercury vapor discharge lamps, mercury constitutes the primarycomponent for the (efficient) generation of ultraviolet (UV) light. Aluminescent layer comprising a luminescent material (for example, afluorescent powder) may be present on an inner wall of the dischargevessel to convert UV to other wavelengths, for example, to UV-B and UV-Afor tanning purposes (sun panel lamps) or to visible radiation forgeneral illumination purposes. Such discharge lamps are therefore alsoreferred to as fluorescent lamps. The discharge vessel of low-pressuremercury vapor discharge lamps is usually tubular and circular in sectionand comprises both elongated and compact embodiments. Generally, thetubular discharge vessel of so-called compact fluorescent lampscomprises a collection of relatively short straight parts having arelatively small diameter, which straight parts are connected togetherby means of bridge parts or arc-shaped parts. Compact fluorescent lampsare usually provided with an (integrated) lamp cap.

In the description and claims of the current invention, the designation“nominal operation” is used to refer to operating conditions where themercury-vapor pressure is such that the radiation output of the lamp isat least 80% of that during optimum operation, i.e. under operatingconditions where the mercury-vapor pressure is optimal. In addition, inthe description and claims, the “initial radiation output” is defined asthe radiation output of the discharge lamp 1 second after switching onthe discharge lamp, and the “run-up time” is defined as the time neededby the discharge lamp to reach a radiation output of 80% of that duringoptimum operation.

It is known that measures are taken in low-pressure mercury vapordischarge lamps to inhibit blackening of parts of the inner wall of thedischarge vessel, which parts are in contact with a discharge which,during operation of the discharge lamp. Such blackening, which isbrought about by interaction between mercury and the material from whichthe inner wall of the discharge vessel is made, is undesirable and doesnot only lead to a reduction of the maintenance but also to anunaesthetic appearance of the lamp, particularly because the blackeningoccurs irregularly, for example, in the form of dark stains or dots.

A low-pressure mercury vapor discharge lamp of the type described in theopening paragraph is known from U.S. Pat. No. 4,544,997. In the knowndischarge lamp, an oxide selected from the group formed by yttrium,scandium, lanthanum, gadolinium, ytterbium and lutetium is used as thetranslucent layer. The oxide is provided as a thin layer on the innerwall of the discharge vessel. The known translucent layers arecolorless, hardly absorb UV radiation or visible light and satisfy therequirements with respect to light and radiation transmissivity. The useof the known translucent layers causes blackening and discoloring of theinner wall of the discharge vessel of the low-pressure mercury vapordischarge lamp to be reduced.

A drawback of the use of the known low-pressure mercury vapor dischargelamp is that the maintenance still is relatively poor due to saidblackening. As a result, in addition, a relatively large amount ofmercury is necessary for the known lamp in order to realize asufficiently long service life. In the case of injudicious processingafter the end of the service life, this is detrimental to theenvironment.

It is an object of the invention to eliminate the above disadvantagewholly or partly. According to the invention, a low-pressure mercuryvapor discharge lamp according to the invention is characterized in thatthe discharge vessel is provided with a pinched seal, and in that thetranslucent layer comprises a borate and/or a phosphate of an alkalineearth metal and/or of scandium, yttrium or a further rare earth metal. Adischarge vessel of a low-pressure mercury vapor discharge lampaccording to the invention having a pinched seal and comprising atransparent layer including said borate and/or phosphate appears to bevery well resistant to the action of the mercury-rare gas atmospherewhich, in operation, prevails in the discharge vessel of thelow-pressure mercury vapor discharge lamp. As a result, blackening dueto interaction between mercury and the glass from which the dischargevessel is manufactured is reduced, resulting in an improvement of themaintenance. During the service life of the discharge lamp, a smallerquantity of mercury is withdrawn from the discharge, so that, inaddition, a reduction of the mercury consumption of the discharge lampis obtained and in the manufacture of the low-pressure mercury vapordischarge lamp a smaller mercury dose will suffice.

Wall blackening caused by withdrawing mercury from the discharge occursin straight parts as well as in arc-shaped parts of low-pressure mercuryvapor discharge lamps and in the sealing areas of the discharge vessel.In the known discharge lamp, the means for maintaining a discharge inthe discharge space are electrodes. The electrodes are supported by an(indented) end portion (also called “stem”) of the discharge vessel.Current supply conductors issue from each electrode through the endportions of the discharge vessel to the exterior. In order to obtain aproper seal when mounting the end portions, it is necessary in the knowndischarge vessel to clean the discharge vessel in the vicinity of theend portions from coatings present on the inside of the dischargevessel. The phosphor coating is normally removed from the sealing areas(end portion(s)) as well as protective coatings made of aluminaparticles. As a consequence, said parts of the discharge vessel in thevicinity of the end portions are sensitive to an attack by the mercuryatmosphere in the discharge lamp, during operation, and substantial wallblackening occurs in the discharge vessel in the vicinity of the endportions. By applying a protective translucent layer according to theinvention in combination with a pinched seal in accordance with theinvention causes blackening to be substantially reduced in the parts ofthe discharge vessel in the vicinity of the end portions. In principle,the entire inner wall surface of the discharge vessel is coated with theprotective translucent layer thereby preventing wall blackening of thedischarge vessel. Advantage of the use of the translucent layeraccording to the invention is that the materials can also be applied atthe part of the wall of the discharge vessel where, during manufacturingof the low-pressure mercury vapor discharge lamp pinched seal is formed.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that, the pinched sealcomprises material from the translucent layer. Because it is no longernecessary to clean the discharge vessel in the vicinity of the pinchedseal (apart from removing the luminescent material), material from thetranslucent layer can be found in the pinched seal.

Another preferred embodiment of the low-pressure mercury vapor dischargelamp according to the invention is characterized in that, the means formaintaining a discharge comprises an electrode pair arranged in thedischarge space and that current supply conductors issue from theelectrode pair through the pinched seal of the discharge vessel to theexterior. In this embodiment the pinched seal also functions as feedthrough for the current supply conductors.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that, the translucentlayer comprises an alkaline earth borate, and in that the thickness ofthe translucent layer is in the range from 0.1-50 μm. By employing atranslucent layer of alkaline earth borate and with a thickness in therange given above appears to be very well resistant to the action of themercury-rare gas atmosphere which, in operation, prevails in thedischarge vessel of the low-pressure mercury vapor discharge lamp. Theinventors have had the insight that by using a suspension of“nano-particles” of alkaline earth borates, in particular calcium,strontium and/or barium borate, a translucent layer can be made with athickness which can be significantly larger than that of the translucentlayer made out of a solution of the salts in the known discharge lamp.With “nano-particles” in the description of the present invention it ismeant that particles with a particle size in the range from 0.1-1 μm.The softening point of the calcium, strontium and/or barium borateparticulate material is low enough that the particles melt togetherduring the bending glass shaping. In addition, a dense translucent layeris obtained that, because of its large thickness, has not completelyreacted with the underlying wall of the discharge vessel in the bentsand in the seal. In experiments it was found that a translucent layermade from nano-particles of calcium, strontium and/or barium borateshowed a relatively high point of zero charge and a relatively lowmercury consumption. An additional advantage of producing thetranslucent layer from nano-particles of alkaline earth borates is thatthe size of the particles of alkaline earth borates is comparable to thewavelength of the UV light. This makes it possible to employ thetranslucent layer also as a reflector for UV light (the size of theparticles is in the range from approximately 0.3 μm to approximately 0.6μm). Preferably, the translucent layer comprises SrB₄O₇. Preferably,nano-particles of SrB₄O₇ with a particle size in the range fromapproximately 0.1 to approximately 1 μm are used to manufacture thetranslucent layer according to the invention.

Preferably, the thickness of the translucent layer is in the range from10-20 μm. Upon making the translucent layer thinner than approximately10 μm could, in particular during bending glass shaping of dischargevessels under factory conditions, give rise to a possible completereaction of the particulate calcium, strontium and/or barium borate withthe wall. The risk is higher in a production environment where theconditions can not always be met as precisely as in laboratoryexperiments. It is observed that in the straight parts of the dischargevessel of compact fluorescent lamps, the particles in the translucentlayer do not reach a high enough temperature to melt leading to diffusescattering of light in the translucent layer. In the arc-shaped parts ofthe discharge vessel of compact fluorescent lamps, the particles in thetranslucent layer reach a high enough temperature to melt leading to atransparent layer.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that the discharge vesselis made from a glass comprising silicon dioxide and sodium oxide, withthe glass composition comprising the following essential constituents,given in percentages by weight (wt. %): 60-80 wt. % SiO₂ and 10-20 wt. %Na₂O. The application of a sealed pinch and a translucent layeraccording to the invention in combination with the sodium-rich glass inaccordance with the invention causes blackening to be substantiallyreduced in the vicinity of the pinch of the discharge vessel. Theinvention is in particular embodied in a combination of a dischargevessel with a pinched pin seal, a coating comprising the borate and/orphosphate as described above and sodium-rich glass.

Sodium-rich glasses are comparatively inexpensive. In the knowndischarge lamp use is made of a so-called mixed alkali glass having acomparatively small SiO₂ content (approximately 67% as compared toapproximately 72% for the sodium-rich glass) and comprising, inter alia,approximately 8 wt. % Na₂O and 5 wt. % K₂O. The cost price of said glassis comparatively high. A comparison between the composition of the knownglass and the sodium-rich glass shows that the alkali content isdifferent. The sodium-rich glass has a comparatively low potassiumcontent, while the known glass is a so-called mixed alkali glass havingan approximately equal molar ratio of Na₂O and K₂O. An advantage residesin that the mobility of the alkali ions in the sodium-rich glass iscomparatively high with respect to the mobility in the mixed alkaliglass. The run-up time for low-pressure mercury vapor discharge lampsmade from sodium-rich glass is approximately the same as for dischargevessels made from the known mixed alkali glass.

The translucent layer in the low-pressure mercury-vapor discharge lampin accordance with the invention further satisfies the requirements withrespect to light and radiation transmissivity and can be easily providedas a very thin, closed and homogeneous translucent layer on an innerwall of a discharge vessel of a low-pressure mercury vapor dischargelamp. This is effected, for example, by rinsing the discharge vesselwith a solution of a mixture of suitable metal-organic compounds (forexample, acetonates or acetates, for example, scandium acetate, yttriumacetate, lanthanum acetate or gadolinium acetate mixed with calciumacetate, strontium acetate or barium acetate) and boric acid orphosphoric acid diluted in water, while the desired translucent layer isobtained after drying and sintering.

A preferred embodiment of the low-pressure mercury vapor discharge lampaccording to the invention is characterized in that a side of thetranslucent layer facing the discharge space is provided with a layer ofa luminescent material. An advantage of the use of a translucent layeraccording to the invention in low-pressure mercury vapor discharge lampsis that the luminescent layer comprising a luminescent material (forexample, a fluorescent powder) adheres significantly better to such atranslucent layer than to a translucent layer of the known low-pressuremercury vapor discharge lamp. Said improved adhesion is obtainedparticularly in the arc-shaped parts of low-pressure mercury-vapordischarge lamps.

The measure according to the invention is notably suitable for compactfluorescent lamps having arc-shaped lamp parts, wherein the dischargevessel is additionally surrounded by a light-transmitting envelope. Thetemperature of the discharge vessel of such “covered” compactfluorescent lamps is comparatively high because the heat dissipation tothe environment is reduced by the presence of the outer envelope. Thisunfavorable temperature balance adversely affects the maintenance of theknown discharge lamp due to an increased level of blackening. Inexperiments it has surprisingly been found that the maintenance of acompact fluorescent lamp provided with a low-pressure mercury vapordischarge lamp according to the invention, the discharge vessel of whichis surrounded by an envelope, has 90% maintenance after 12,000 burninghours, while the maintenance of an identical compact fluorescent lampprovided with the known low-pressure mercury vapor discharge lamp, thedischarge vessel of which is surrounded by an envelope, is less than 80%after 12,000 burning hours and fluctuates (depending on the amount of Hgconsumption consumed). The depletion of mercury out of the amalgam canbe so high that the amalgam does no longer give the optimum mercurypressure. In addition, the light output drops significantly.

The glass composition preferably includes the following constituents:70-75 wt. % SiO₂, 15-18 wt. % Na₂O, and 0.25-2 wt. % K₂O. Thecomposition of such a sodium-rich glass is similar to that of ordinarywindow glass and it is comparatively cheap with respect to the glassused in the known discharge lamp. The cost price of the raw materialsfor the sodium-rich glass as used in the discharge lamp in accordancewith the invention is only approximately 50% of the cost price of theraw materials for the mixed alkali glass as used in the known dischargelamp. Moreover, the conductance of said sodium-rich glass iscomparatively low; at 250° C. the conductance is approximately log ρ=6.3while the corresponding value of the mixed alkali glass is approximatelylog ρ=8.9.

In a preferred embodiment of the low-pressure mercury vapor dischargelamp according to the invention, the translucent layer comprises aborate and/or a phosphate of calcium, strontium and/or barium. Such atranslucent layer has a relatively high coefficient of transmission forvisible light. Moreover, low-pressure mercury vapor discharge lamps witha translucent layer comprising calcium borate, strontium borate orbarium borate or calcium phosphate, strontium phosphate or bariumphosphate have a good maintenance.

In a particularly preferred embodiment of the low-pressure mercury vapordischarge lamp according to the invention, the translucent layercomprises an yttrium-strontium-borate composition. Such a translucentlayer has a relatively high coefficient of transmission for ultravioletradiation and visible light. It has further been found that atranslucent layer comprising yttrium borate and strontium borate is onlyslightly hygroscopic and adheres well to the inner wall of the dischargevessel. Moreover, the layer can be provided in a relatively simplemanner (for example, with yttrium acetate and strontium acetate mixedwith boric acid), which has a cost-saving effect, notably useful in amass manufacturing process for low-pressure mercury vapor dischargelamps.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1A is a cross-sectional view of an embodiment of a compactfluorescent lamp comprising a low-pressure mercury vapor discharge lampaccording to the invention, and

FIG. 1B is a cross-sectional view of a detail of the low-pressuremercury vapor discharge lamp as shown in FIG. 1A.

The Figures are purely diagrammatic and not drawn to scale. Particularlyfor clarity, some dimensions are exaggerated strongly. Similarcomponents in the Figures are denoted by the same reference numerals asmuch as possible.

FIG. 1A shows a compact fluorescent lamp comprising a low-pressuremercury vapor discharge lamp. The low-pressure mercury-vapor dischargelamp is provided with a radiation-transmitting discharge vessel 10enclosing a discharge space 11 having a volume of approximately 10 cm³.The discharge vessel 10 is a glass tube which is at least substantiallycircular in cross-section and the (effective) internal diameter of whichis approximately 10 mm. The discharge vessel 10 is closed in a gastightmanner by a pinched seal 20 according to the invention (see FIG. 1B).The pinched seal 20 is made by press sealing. The tube is bent in theform of a so-called hook and, in this embodiment, it has a number ofstraight parts, two of which, referenced 31, 33, are shown in FIG. 1A.The discharge vessel further comprises a number of arc-shaped parts, twoof which, referenced 32, 34, are shown in FIG. 1A. An inner wall 12 ofthe discharge vessel 10 is provided with a translucent layer 16 and witha luminescent layer 17. In an alternative embodiment, the luminescentlayer has been omitted. The use of a pinched seal 20 and the applicationof the bendable translucent layer 16 according to the invention enablesthe entire surface area of the inner wall 12 of the discharge vessel 10to be coated with the protective translucent layer 16. The inventivecombination of the pinched seal 20 and the application of the bendabletranslucent layer 16 according to the invention allows the use ofsodium-rich glass as material for the discharge vessel. Particularlypreferred is a glass of the following composition: 70-74 wt. % SiO₂,16-18 wt. % Na₂O, 0.5-1.3 wt. % K₂O, 4-6 wt. % CaO, 2.5-3.5 wt. % MgO,1-2 wt. % Al₂O₃, 0-0.6 wt. % Sb₂O₃, 0-0.15 wt. % Fe₂O₃ and 0-0.05 wt. %MnO. Excellent run-up characteristics are obtained for low-pressuremercury vapor discharge lamps made from sodium-rich glass.

The discharge vessel 10 is supported by a housing 70 which also supportsa lamp cap 71 provided with electrical and mechanical contacts 73 a, 73b, which are known per se. The discharge vessel 10 of the low-pressuremercury-vapor discharge lamp is surrounded by a light-transmittingenvelope 60 which is attached to the lamp housing 70. Thelight-transmitting envelope 60 generally has a matt appearance.

FIG. 1B very diagrammatically shows a cross-sectional view of a detailof the low-pressure mercury-vapor discharge lamp shown in FIG. 1A. Thedischarge space 11 in the discharge vessel 10 does not only comprisemercury but also a rare gas, argon in this example. Means formaintaining a discharge are constituted by an electrode pair 41 a (onlyone electrode is shown in FIG. 1B) which is arranged in the dischargespace 11. In an alternative embodiment the low-pressure mercury vapordischarge lamp is a so-called electrode-less discharge lamp. Theelectrode 41 a in FIG. 1B is a winding of tungsten coated with anelectron-emissive material, here a mixture of barium oxide, calciumoxide and strontium oxide. Current supply conductors 50 a, 50 a′ issuefrom the electrode pair 41 a through the pinched seal 20 end portions ofthe discharge vessel 10 to the exterior. The electrode 41 a is supportedby the pinched seal 20 which seal closes the discharge vessel 10 in agastight manner. The current supply conductors 50 a, 50 a′ are connectedto an (electronic) power supply which is accommodated in the housing 70and electrically connected to the electrical contacts 73 b at the lampcap 71 (see FIG. 1A).

In an embodiment of the low-pressure mercury vapor discharge lamp,various concentrations of an Sr(Ac)₂ (strontium acetate) solution andH₃BO₃ (boric acid) are added to solutions comprising variousconcentrations of Y(Ac)₃ (yttrium acetate) to manufacture thetranslucent layer 16 according to the invention. In an alternativeembodiment, a Ba(Ac)₂ (barium acetate ) solution is added instead of anSr(Ac)₂ solution. Three recipes were tested, as shown in Table I. TABLEI Three recipes for a translucent layer. Recipe wt. % Y(Ac)₃ mol Sr(Ac)₂mol H₃BO₃ R₁ 0.11 0.036 0.147 R₂ 0.15 0.06 0.24 R₃ 0.15 0.048 0.191

Before coating, the discharge vessels were bent in the known hook shapehaving straight parts and arcuate parts. In an alternative embodiment,the bending took place after coating the discharge vessel. After rinsingand drying, the discharge vessels were provided with a coating bypassing an excess of the afore-mentioned solutions through the dischargevessels. After said coating operation, the discharge vessels were firstdried in air at a temperature of approximately 60° C. for 15 minutes andsubsequently sintered at approximately 550° C. for 2 minutes. In analternative embodiment, the translucent coating is fixed in a shorterperiod of time at a higher temperature.

In a preferred embodiment of the low-pressure mercury vapor dischargelamp, so-called nano-particles of SrB₄O₇ with a particle size in therange from approximately 0.1 to approximately 1 μm are used tomanufacture the translucent layer 16 according to the invention.Stoichiometric quantities of SrCO₃ and H₃BO₃ are mixed and melted in aPt-crucible in air. After cooling down, the glass is crushed and milledwith butyl acetate during two hours followed by 48 hours rolling withZrO₂ spheres. The resulting amorphous particles of SrB₄O₇ have anaverage particle size of 0.6 μm. After proving the discharge vesselswith such a coating, the discharge vessels were first dried in air at atemperature of approximately 60° C. for 15 minutes. In an alternativeembodiment, the transparent coating is fixed in a shorter period of timeat a higher temperature. The thickness of the translucent layer 16ranges from approximately 1 μm to approximately 50 μm, preferably fromapproximately 10 μm to approximately 20 μm. In an alternativeembodiment, nano-particles of BaB₄O₇ or CaB₄O₇ are employed.

Subsequently, the discharge vessels were provided with a luminescentcoating comprising three known phosphors, namely a green-luminescentmaterial with terbium-activated cerium magnesium aluminate, ablue-luminescent material with bivalent europium-activated bariummagnesium aluminate, and a red-luminescent material with trivalenteuropium-activated yttrium oxide. A number of said discharge vesselswere subsequently assembled to low-pressure mercury vapor dischargelamps in the customary manner. A number of these discharge lamps weresubsequently provided with a translucent envelope on the basis of one ofthe three recipes mentioned hereinabove (see the example shown in FIG.1A). Experiments were carried out on discharge vessels of two lengths,namely 230 mm (11W fluorescent lamp) and 405 mm (20W fluorescent lamp).The current intensity of the lamp during operation was 200 mA in allcases.

Subsequently, the maintenance after 1,000 hours has been measured oflow-pressure mercury-vapor discharge lamps comprising a discharge vesselin accordance with the invention and provided with the R3 composition ofthe translucent layer in accordance with the invention. For comparison,the maintenance of discharge vessels with the standard seal and atransparent layer of known yttrium oxide is given. The results of thesemeasurements are shown in Table II. TABLE II Maintenance data (1000hours) of low-pressure mercury-vapor discharge lamps comprising adischarge vessel with a pinched seal and made from sodium-rich glass andprovided with the R3 composition of the translucent layer in accordancewith the invention. For comparison, the maintenance of discharge vesselswith the standard seal and a transparent layer of known yttrium oxide isgiven. Known glass Sodium-rich glass No pinched seal With pinched sealKnown Y₂O₃ translucent Translucent layer from layer R3 composition Nopinched seal 95 (4) 66 (18) With pinched seal 95 (4) 95 (6) 

Table II shows that after 1,000 hours the maintenance of discharge lampscomprising the discharge vessel with a pinched seal and made fromsodium-rich glass and provided with the translucent layer according tothe invention is relatively high. Up to 12,000 hours there is nosignificant difference in maintenance with the known glass and without apinched seal between the three compositions of the translucent layer inaccordance with the invention.

In Table III the amount of bound mercury (in μg) in the discharge vesselafter 1000 hours life time of low-pressure mercury-vapor discharge lampscomprising a discharge vessel with a pinched seal and made fromsodium-rich glass and provided with the R3 composition of thetranslucent layer (see Table I). For comparison, the date for dischargevessels with the standard seal are given. TABLE III Bound mercury (Hg)in the discharge vessel after 1000 hours life time of low-pressuremercury-vapor discharge lamps comprising a discharge vessel with apinched seal and made from sodium-rich glass and provided with the R3composition of the translucent layer in accordance with the invention.For comparison, the date for discharge vessels with the standard seal isgiven. Known glass Sodium-rich glass Sodium-rich glass No pinched sealNo pinched seal With pinched seal Known Y₂O₃ Translucent layer fromTranslucent layer from translucent layer R3 composition R3 composition110 μg Hg 922 μg Hg 100 μg Hg

The relatively high Hg consumption of the discharge vessel made ofsodium rich glass and without a sealed pinch is mainly located in theseal area.

It will be evident that within the scope of the invention manyvariations are possible to those skilled in the art.

The scope of protection of the invention is not limited to the examplesgiven herein. The invention is embodied in each novel characteristic andeach combination of characteristics. Reference numerals in the claims donot limit the scope of protection of the claims. The word “comprising”does not exclude the presence of elements other than those mentioned inthe claims. The use of the word “a” or “an” in front of an element doesnot exclude the presence of a plurality of such elements.

1. A low-pressure mercury vapor discharge lamp comprising alight-transmitting discharge vessel, the discharge vessel enclosing, ina gastight manner, a discharge space provided with a filling of mercuryand a rare gas, the discharge vessel comprising means for maintaining adischarge in the discharge space, while at least a part of an inner wallof the discharge vessel is provided with a translucent layer,characterized in that the translucent layer comprises a borate and/or aphosphate of an alkaline earth metal and/or of scandium, yttrium or afurther rare earth metal, and in that the discharge vessel is providedwith a pinched seal.
 2. A low-pressure mercury vapor discharge lamp asclaimed in claim 1, characterized in that the pinched seal comprisesmaterial from the translucent layer.
 3. A low-pressure mercury vapordischarge lamp as claimed in claim 1, characterized in that the meansfor maintaining a discharge comprises an electrode pair arranged in thedischarge space and that current supply conductors issue from theelectrode pair through the pinched seal of the discharge vessel to theexterior.
 4. A low-pressure mercury vapor discharge lamp as claimed inclaim 1, characterized in that the translucent layer comprises analkaline earth borate, and in that the thickness of the translucentlayer is in the range from 0.1-50 μm.
 5. A low-pressure mercury vapordischarge lamp as claimed in claim 4, characterized in that thetranslucent layer comprises SrB₄O₇.
 6. A low-pressure mercury vapordischarge lamp as claimed in claim 4, characterized in that thethickness of the translucent layer is in the range from 10-20 μm.
 7. Alow-pressure mercury vapor discharge lamp as claimed in claim 1,characterized in that the discharge vessel is made from a glasscomprising silicon dioxide and sodium oxide, with the glass compositioncomprising the following essential constituents, given in percentages byweight (wt. %):60-80 wt. % SiO₂,10-20 wt. % Na₂O.
 8. A low-pressure mercury vapor discharge lamp asclaimed in claim 8, characterized in that the glass composition includesthe following constituents:70-75 wt. % SiO₂,15-18 wt. % Na₂O,0.25-2 wt. % K₂O.
 9. A low-pressure mercury vapor discharge lamp asclaimed in claim 1, characterized in that a side of the translucentlayer facing the discharge space is provided with a layer of aluminescent material.
 10. A compact fluorescent lamp comprising alow-pressure mercury-vapor discharge lamp as claimed in claim 1,characterized in that a lamp housing is attached to the discharge vesselof the low-pressure mercury-vapor discharge lamp, which lamp housing isprovided with a lamp cap.
 11. A compact fluorescent lamp as claimed inclaim 10, characterized in that the discharge vessel of the low-pressuremercury-vapor discharge lamp is surrounded by a light-transmittingenvelope which is attached to the lamp housing.